This invention relates to an aluminum nitride circuit substrate, more particularly, to an aluminum nitride circuit substrate being free from generation of cracks due to, for example, a large bonding area between a lead frame and a substrate.
In the prior art, as a circuit substrate for a semiconductor, there has been generally employed an inexpensive alumina (Al.sub.2 O.sub.3) substrate, on which a circuit is formed and various elements including a semiconductor device are mounted on the circuit.
However, in recent years, with the tendency that the semiconductor device have increase output, a heat release value of such devices become higher. Therefore, as a result the above-mentioned Al.sub.2 O.sub.3 substrate is not necessarily sufficient in the point of the heat dissipating characteristic. Also, with the above tendency, a lead frame has become larger in size.
For this reason, the present inventors have tried to obtain a circuit substrate employing an aluminum nitride (AlN), having an excellent heat dissipating characteristic instead of Al.sub.2 O.sub.3. However, the circuit substrate of AlN tends to cause the following new problem. That is, on a predetermined position of a surface of the circuit substrate, the so-called lead frame functioning as an electrode as well is bonded. In the prior art, a conventional metallized layer is formed on a surface of the Al.sub.2 O.sub.3 substrate and then an electroconductive substance to be used as the lead frame is bonded through the metallized layer. In this case, Fe-Ni-Co alloy has been used for hermetic sealing from a viewpoint of readiness in soldering property.
However, in such an AlN circuit substrate prepared by forming the conventional metallized layer on the surface of the AlN circuit substrate and bonding the lead frame, sometimes the generation of cracks is observed at the bonded portion with the increase of a temperature.